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To procreate, low-ranking males use alternative strategies

Lower-ranking male chimpanzees are at a disadvantage when it comes to competing directly with other males for access to fertile females, but they can compensate with alternative mating strategies, a new study out of Gombe National Park in Tanzania shows.

Lead researcher Emily Wroblewski analyzed data that Gombe researchers collected from 1984 to 2005. She wanted to know if higher-ranking male chimps would be more successful at reproducing (because they have the greatest access to females), as is the case in many species.

Wroblewski determined paternity using DNA samples collected noninvasively, from fecal samples on the forest floor. She then applied a reproductive model to Gombe data factoring in male rank, number of male competitors and number of receptive females. (This is called the “priority access model.”) The results showed that the highest-ranking males had greatest reproductive success, as predicted by the model.

Opportunities seized

But there was a surprise. The lowest-ranking males did not do as poorly as the model predicted. Analysis of mating strategies showed that these fathers managed to reproduce by courting less desirable females – generally younger females who had less experience as mothers. And, the lower-ranking fathers took the females more often on “consortships.” In a consortship, a male takes a female to a remote part of the chimpanzee range to mate with her free from interference from other males. Consortships can last two or three weeks.

“Rank is important, but other males do have opportunities and success outside of what we would predict based on their rank,” says Wroblewski.

In sum, the study confirmed that high-ranking males do have the most offspring. But other factors are important – a male’s choice of mate and use of alternative mating strategies.

Further questions

The study also showed that # 3 and # 4 ranked males do not fare as well as a straight application of the model would predict. Exploring why these still fairly high-ranking males get less success than predicted is one of the future directions of this study. They are less likely to use alternative mating strategies like consortship, and they may face more aggression from the dominant, alpha male preventing them from mating with females.

The study was published in the February edition of Animal Behaviour. The research team included Wroblewski, Joann C. Schumacher-Stankey and Anne E. Pusey of the Jane Goodall Institute’s Center for Primate Studies at the University of Minnesota, Carson M. Murray of Lincoln Park Zoo, Chicago, along with Brandon F. Keele and Beatrice H. Hahn (University of Alabama-Birmingham).

Wroblewski will also use the knowledge of who the fathers are to look at whether fathers treat male and female offspring differently than other chimpanzees. For example, do fathers avoid mating with daughters? Do they help sons work their way up the dominance hierarchy?

Wroblewski is a doctoral student at the University of Minnesota and studied chimps at Gombe from 2005 to 2007.